1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD in which a light guiding plate is removed from an LCD panel and a polarizing plate is placed in the LCD panel.
2. Description of the Related Art
Generally, LCDs are widely being used as portable information terminal because of their slimmer in thickness and lighter in weight characteristics. An LCD panel is a passive device that does not emit light by itself, and is provided with a reflection plate at a rear side thereof. As such a LCD panel, there are a reflection type LCD panel that uses an external light as a light source, a projection type LCD panel, and a semi-projection type LCD panel. Each of the projection type LCD panel and the semi-projection type LCD panel has a back light assembly at a rear side thereof.
Since the LCD panel can be driven at a very low voltage, for instance, a few volts, the reflection type LCD has a low power consumption characteristic, but it cannot be used in a dark environment. Also, since a color LCD panel using a color filter has a very low efficiency of light, there is a drawback in that the reflective type LCD cannot display bright colors.
Regardless of the used light source types, for instance, natural light source or artificial light source, the light vibrating along only one direction is used. To this end, a polarizing plate is attached to both outer surfaces of the LCD panel.
Light is a kind of electromagnetic wave, vibrates in a sine function, and has a vibration direction normal to an advancing direction thereof. This vibration direction exists at the same probability in any direction without any directionality. The polarizing plate passes only the light vibrating along a selected one direction, and absorbs or reflects the remaining light vibrating along unselected directions using a proper medium. Resultantly, the polarizing plate functions to make a light vibrating along a specific direction.
In order to enhance the light efficiency, the polarizing plate is attached to both outer surfaces of the LCD panel. As these polarizing plates, there are a urea-based polarizing plate and a dye-based polarizing plate.
The urea-based polarizing plate is prepared by allowing polyvinyl alcohol to adsorb an urea-based solution having a high anisotropy, and drawing the resultant polyvinyl alcohol along a specific direction. The urea-based film has advantages such as a superior polarization property, and a high contrast ratio, but it may cause a problem due to a high sublimation of iodine molecules under an environment having a high temperature and a high humidity.
The dye-based polarizing plate is made by a similar method to the manufacturing method of the urea-based polarizing plate, i.e., by allowing polyvinyl alcohol as a polarization medium to adsorb a dye having a dichroism, and drawing the resultant polyvinyl alcohol such that chloride molecules are arranged along the drawing direction. As kinds of dye used, there are a direct dye and an acid dye. This dye-based polarizing plate, however, has a low polarity because of a low anisotropy of the dye itself compared with the urea-based polarizing plate, but it has a strong resistance against an environment having a high temperature and a high humidity.
FIG. 1 is a schematic view showing a conventional LCD in which a polarizing plate is used. Referring to FIG. 1, polarizing plates 40 and 50 are respectively attached to outer surfaces of upper glass substrate and lower glass substrate 10 and 20. A backlight assembly 90 is arranged near a side surface of a light guiding plate disposed below the lower glass substrate 20. A liquid crystal layer 30 is interposed between the upper glass substrate 10 and the lower glass substrate 20.
In FIG. 1, a reference numeral 32 denotes a sealant, a reference numeral 80 denotes an LCD panel.
FIG. 2 is a sectional view showing a constitution of the polarizing plate used in the LCD of FIG. 1. Referring to FIG. 2, a polymer polarization medium of polyvinyl alcohol layer 43 is centered at the polarizing plate, and supporter layers 42 and 44 of triacetyl cellulose (TAC) are attached on both surfaces of the polyvinyl alcohol layer 43. An upper protection film 41 is attached on the upper supporter layer 42, and an adhesive layer 45 and a lower protection film 46 in the order named are attached on the lower supporter layer 44.
Here, the supporter layers 42 and 44 are used for securing durability, mechanical strength, heat resistant property, humidity resistant property, etc., and the adhesive layer 45 is used for adhering the polarizing plates 40 and 50 to the glass substrates 10 and 20.
Among the aforementioned elements, the light guiding plate and the polarizing plate are important factors determining the whole thickness of the LCD panel.
Therefore, it is necessary to decrease the total thickness of the LCD panel by decreasing the thickness of the light guiding plate and the polarizing plate.